1. Field of Invention
The invention relates to a method for growing p-type SiC semiconductor single crystals, and also relates to p-type SiC semiconductor single crystals obtained by this method.
2. Description of the Related Art
SiC semiconductors have excellent properties, such as a higher withstand voltage than that of Si semiconductors, resistance to high temperatures, and reduced power loss or dissipation, and thus have been developed for practical use as high-voltage, large-current power semiconductors suitable for, for example, inverters. For example, MOSFETs (metal oxide semiconductor field effect transistors) and IGBTs (insulated gate bipolar transistors) that provide switching devices of inverters are required to have small ON resistance and high switching speed. Of n-type and p-type SiC semiconductors that constitute these transistors, n-type semiconductors are currently at a relatively advanced stage of development, whereas the development of p-type semiconductors is delayed, thus making it difficult or impossible to achieve low resistance suitable for practical use.
Typical examples of techniques for producing p-type SiC semiconductors include, for example, a sublimation process, ion-implantation, and liquid phase deposition or solution method. The sublimation process, which has been most widely used, provide wafers of, for example, 6H—SiC doped with B or Al, which have a resistivity of 2 to 10 Ωcm as experimental values, but commercial products actually manufactured by this process have a resistivity of 200 Ωcm or higher. While the practical level of the resistivity required to be possessed by the above-described switching devices is 0.5 Ωcm or lower, the resistivity of the SiC semiconductors produced by the sublimation process does not reach the practical level. This may be because 1) doping of SiC semiconductors with an impurity such as B or Al (i.e., introduction of an impurity, such as B or Al, into SiC semiconductors) is difficult, or (2) the activation energy of 4H—SiC is equal to 285 meV when it is doped with B and is equal to 190 meV when it is doped with Al (“Foundations and Applications of SiC Devices” edited by Kazuo ARM and published in 2003 by Ohmsha); because of such a large activation energy, activated carriers are less likely to be produced, i.e., the rate of generation of activated carriers is low.
By ion implantation, on the other hand, 6H—SiC, when doped at an impurity concentration of 1020 to 1021 cm−3, provides an extremely low resistivity of 0.02 Ωcm (“Technology of Semiconductor SiC and Its Application” by Hiroyuki MATSUNAMI, published in 2003 by Nikkan Kogyo Shimbun Ltd.). If such a high impurity concentration or large doping amount is employed, crystal defects are inevitably generated, and high-quality semiconductor single crystals cannot be obtained. Also, the activation energy of 4H—SiC is equal to 285 meV when it is doped with B and is equal to 190 meV when it is doped with Al (“Foundations and Applications of SiC Devices” edited by Kazuo ARAI and published in 2003 by Ohmsha); because of such a large activation energy, activated carriers are less likely to be produced, i.e., the rate of generation of activated carriers is low, as is the case with the sublimation process.
The liquid phase deposition or solution method is conducive to an improvement in the polytype controllability and reduction of micropipes. Thus, various proposals have been made on this method.
Japanese Patent Application Publication No. 2000-264790 (JP-A-2000-264790) discloses a p-type SiC semiconductor single crystal obtained by adding Al or B into a solution in which C is dissolved in a melt of Si (which solution will be simply called “Si—C solution”), and an n-type SiC semiconductor single crystal similarly obtained by adding N to the Si—C solution. However, this publication does not particularly address reduction of the resistance.
In Japanese Patent Application Publication No. 2007-76986 (JP-A-2007-76986), it is proposed to grow a SiC semiconductor single crystal by adding Ti and Al to a Si—C solution. However, this publication does not particularly address reduction of the resistance.
In Japanese Patent Application Publication No. 2005-82435 (JP-A-2005-82435), it is proposed to improve the flatness of a growth surface of a SiC single crystal by adding 1 to 30 wt % of Al to a Si—C solution. However, this publication does not mention the amount of Al contained in the resultant SiC single crystal nor addresses reduction of the resistance.
In Japanese Patent Application Publication No. 2007-153719 (JP-A-2007-153719), it is proposed to deposit N on a porous graphite crucible so that N is add into a Si—C solution. Since N is an n-type dopant, the addition of only N does not result in production of a p-type SiC semiconductor single crystal. In this publication, there is no suggestion about reduction of the resistance.